1. Field of the Invention
The present invention is in the field of electromagnetic radiation detection systems. More specifically, the present invention is directed to an improved signal handling technique for use with pyroelectric detectors, in order to reduce the effects of microphonics on the output signal of each detector.
2. Description of the Prior Art
Infrared radiation detection systems, which employ pyroelectric type detector elements, are generally considered to be quite desirable, since they operate at ambient temperatures and do not require cooling. The avoidance or elimination of associated cooling systems is particularly important in most airborne applications, where weight is critical. However, it is recognized that pyroelectric detector elements are susceptible to undesirable noise effects on detected radiation signals. Due to the fact that pyroelectric detectors are piezoelectric crystals, unwanted strains due to vibrations, as well as thermal changes caused by received radiation, may occur. These strains cause unwanted noise signal components to be output from the detector elements and must be filtered out.
An example of a prior art attempt to eliminate unwanted signal components is shown in U.S. Pat. No. 3,453,432. In that patent, each active (exposed to radiation) pyroelectric detector element is employed in combination with a compensating (shaded from radiation) pyroelectric detector element mounted on a common substrate. The active detector element is oriented to be exposed to incoming radiation while the compensating detector is arranged adjacent the active detector, but shielded so as to not be exposed to incoming radiation. The active and compensating detectors are initially oppositely polarized by the application of opposite voltages. Thereafter, the active and compensating detectors are connected in parallel so that undesirable signals generated, due to substrate temperature changes, are cancelled.
Vibrational noise is sought to be decreased by the invention described in U.S. Pat. No. 4,060,729. In this patent, the ambient noises are recognized as being due to both temperature variations and ambient vibrations. Therefore, employing a technique similar to that described above, two detector elements are mounted in an adjacent configuration so that they are exposed to identical temperature and vibrational variations. In a first embodiment, two elements are initially oppositely polarized and then connected in parallel to effect cancellation of the noise signals created by the temperature variations and vibration. An alternative embodiment is also shown, wherein the two detecting elements are initially polarized the same and the outputs are electrically connected in series opposition, to effect thermal and vibrational noise cancellation.
While the above prior art techniques are suitable for some applications, it has been found that most lightweight, compact, airborne design applications do not allow the luxury of redundant compensation detectors in an array of active pyroelectric detectors. In addition, it has also been found that the above discussed prior art techniques, while reducing noise due to thermal and vibrational variation, produce an increase in random noise signal power. This is due to the fact that uncorrelated random noise is generated independently in each separate pyroelectric detector element. Therefore, when the outputs of two detector elements are subtracted, the random noise power is doubled, rather than cancelled.